Monopulse radar systems are used for searching for targets and for tracking targets. In monopulse radar systems, information concerning the angular location of a source or target is obtained by comparison of signals received in two or more simultaneous antenna beams. Referring to FIG. 1, monopulse radar systems have array radar antennas, such as antenna 12, that output antenna voltage data to beamformer 14. In searching for targets, such a system may interrogate a grid of angular positions using a single monopulse transmission beam set formed by beamformer 14. Beamformer 14 forms a detection sum beam, which is output to a processor, and processed using target detection logic 18 to determine whether a target is present. Beamformer 14 also forms orthogonal difference beams, i.e., azimuth difference beam and an elevation difference beam, which may be output to a processor. If target detection logic 18 indicates that a target is detected, the processor may provide an indication to trigger angle processing, and may then determine the angle of arrival of the target using the two orthogonal difference beams. The processor may simultaneously calculate the ratios of the difference beams to the sum beam, thus determining the azimuth and elevation monopulse ratios, and determine, from a look up table, the corresponding azimuth and elevation angles 20. The elevation and azimuth angles may then be output by a processor to a target detection and angle output 22. This process provides an estimate of the angular location of the target in the elevation and azimuth axes. This approach suffers a beam-shape loss (i.e., a decreased signal to noise ratio due to decreased antenna gain), when the target is off the beam steering direction (i.e. off the center of the beam set). As the angular distance of the target from the beam center or boresight increases, the signal to noise ratio decreases. This approach has maximum gain response and, as a result, best angle estimation performance, at the peak of the sum beam.
Monopulse radar systems similarly perform a tracking function on targets of interest. Equivalent angle estimation beam shape loss occurs in tracking targets. The extent of allowable loss affects the revisit rate of the track function. The quality of the tracking and maximum maneuver capability of the target determine how often the track must be revisited to keep the beam steering direction close enough to the actual target to avoid losing the target.
In monopulse radar processing, antenna beamforming is performed to generate the sum beam, the delta-azimuth beam and the delta-elevation beam. A processor uses target detection logic to compare the magnitude of the sum beam to a threshold. Detection is declared when the magnitude of the sum beam output is above the threshold. When a target is detected, the azimuth and elevation monopulse ratios are formed by using the delta-azimuth beam and the delta-elevation beam in addition to the sum beam. The directional cosines (i.e. in u- and v-space) or the corresponding azimuth and elevation angles are then determined by consulting a look-up table or by using a one-dimensional polynomial function. The determination of the directional cosine for the azimuth angle may be expressed as
            u      ^        =                  f                  -          1                    ⁡              (                              I            ⁢            m                    ⁢                      {                                          Δ                A                            ∑                        }                          )              ,where ΔA is the delta-azimuth beam, Σ is the sum beam, and
  Im  ⁢      {                  Δ        A            Σ        }  is the imaginary part of the ratio, or the imaginary part of the azimuth monopulse ratio. It will be appreciated that the real part of the monopulse ratio may also be used depending on the convention used. The function ƒ−1 denotes a look up table to determine the u directional cosine. Similarly, the determination of the directional cosine for the elevation angle may be expressed as
            v      ^        =                  g                  -          1                    ⁡              (                  Im          ⁢                      {                                          Δ                E                            Σ                        }                          )              ,where ΔE is the delta-elevation beam, Σ is the sum beam, and
  Im  ⁢      {                  Δ        E            Σ        }  is the imaginary part of the elevation monopulse ratio. The function g−1 denotes a look up table to determine the v directional cosine. The real part of the monopulse ratio may also be used depending on the convention used.
As noted above, the above approach suffers from beam-shape loss. The effects of beam-shape loss can be ameliorated by transmitting at higher power; however, the use of higher power transmissions will mean the use of excessively high power levels, and thus unnecessary and wasteful use of power, if the target is at the center or the vicinity of the center of the beam. For search, the effects of beam shape loss may be ameliorated by packing the beams closer together; however, packing the beams closer together results in greater use of processing resources or slower searching. For tracking, a higher update rate may be employed to ameliorate the effects of beam shape loss; however, a higher update rate uses more transmission power and processing resources.